1. Field of the Invention
The present invention relates to a lithographic apparatus, an alignment system and an alignment method.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Generally, a number of systems and/or objects are aligned before performing certain steps of a lithographic device manufacturing method. For example, the patterning device, the substrate and the projection beam should be aligned correctly while the pattern is being imaged. Also, other devices should be aligned and/or alignment of a number of devices may be desirable during these and other steps of the lithographic method.
To align objects, an alignment system may be provided. In conventional lithographic apparatus, a docking principle is employed to mechanically align two or more devices. In the docking method, a first device is moved towards a second device. When the first device arrives near the second device, the first device mechanically engages the second device at a number of predetermined positions. Due to the engagement, the relative position of the first device is known with respect to the second device. Depending on a number of characteristics (e.g. the shape of the first and second devices at the engaging locations, the movement of the first device, and/or possibly other characteristics) the relative positioning, i.e. the alignment, of the two devices has a predetermined accuracy.
The alignment method using the docking principle may be performed in combination with other alignment methods. For example, it may be possible to align a device in a first direction using a proximity switch. Then, the device may be aligned in a second and further direction using the docking principle described above.
The first device may be connected to an actuator configured to move the first device with a wrist assembly. Such a wrist assembly provides a flexible coupling between the first device and the actuator, which, among other things, provides flexibility for calibration and alignment purposes.
However, a wrist assembly is a complex structure requiring high manufacturing costs.